The present invention relates to a mechanism for converting rotary motion into linear motion. More particularly, the present invention relates to a scotch yoke assembly that produces two different stroke lengths and is selectively determined by the rotational direction of a cam. The scotch yoke assembly of the present invention is particularly suited for reciprocating a tool, such as a saw blade.
The present invention has applicability to power tools, and in particular to power tools having reciprocating drive mechanisms. Jigsaws and other reciprocating saws (referred to in the trade as xe2x80x9creciproxe2x80x9d saws) are the most common types of power tools having a reciprocating drive mechanism. However, it will be understood that the present invention has general applicability to any power tool that utilizes a reciprocating drive mechanism.
Reciprocating saws of the general type to which the present invention pertains typically include an electric motor arranged to drive a reciprocating saw blade drive mechanism, which may be of the scotch yoke type. More particularly, the motor shaft is mechanically coupled to a crank plate having an eccentrically located crank pin that is drivingly received within the slot of a reciprocating yoke member. The yoke member is coupled to the saw blade plunger sometimes referred to as a ram.
Mechanisms of the type described, although generally satisfactory, do not provide a means for adjusting the stroke of the reciprocating member. As is known to those skilled in the art, it is desirable to be able to vary the stroke of a reciprocating saw blade depending on the nature of the work being cut.
Reciprocating saws and jigsaws are used to cut a variety of materials, including materials which differ widely in density. As is known to those skilled in the art, as a general rule it is desirable when cutting soft materials to provide the saw blade with a rather large stroke, say two inches for purposes of illustration. On the other hand, when cutting hard materials it is desirable to provide the blade with a much shorter stroke, say a stroke of one inch for purposes of illustration. Thus, it is very desirable to provide an adjustable stroke reciprocating mechanism thereby to permit the tool to cut efficiently irrespective of the nature of the material being cut.
Aside from varying the stroke of the saw blade to accommodate materials of varying properties, it is also advantageous to be able to shorten the stroke of the saw blade to be able to cut objects in cramped or confined environments. Minimizing the stroke of the blade in such an instance reduces the possibility of unintended contact between the tip of the blade and other objects in the vicinity of the member being cut.
An adjustable stroke mechanism is disclosed in U.S. Pat. No. 5,134,777. This stroke mechanism utilizes a planetary gear system including a ring gear and an idler gear. The stroke length of a crank pin is adjusted by varying the position of the ring gear relative to the idler gear.
The ""777 patent discloses a relatively flexible arrangement involving a relatively large number of parts including a generally circular actuator plate mounted within a tool housing for rotation about an axis of a drive shaft. The actuator has a number of projections which are received within corresponding recesses in the ring gear thereby connecting the actuator plate to the ring gear. Moreover, the actuator plate includes a generally elliptical opening to accommodate movement of the crank pin. Adjustment of the stroke length of the crank pin is achieved by rotating the actuator plate which causes the crank pin to move along an elliptical path. The stroke mechanism of the ""777 patent is advantageous in that any of a number of discrete stroke lengths can be selected; however, this flexibility comes at the expense of a complex design involving a relatively large number of parts. Consequently, a stroke mechanism according to the ""777 patent is relatively expensive to manufacture.
Accordingly, it is an object of the present invention to provide an improved adjustable stroke mechanism for a scotch yoke assembly which utilizes fewer parts than known scotch yoke assemblies having such stroke length adjust ability.
Another object of the present invention is to provide an improved adjustable stroke mechanism in which one of two stroke lengths is selected in accordance with a rotational direction of a cam.
Yet another object of the present invention is to provide a reciprocating saw with an improved adjustable stroke mechanism for linearly driving a saw blade at one of two lengths in accordance with a rotational direction of a reversible rotary motor.
The above-described objects are met or exceeded by the present adjustable stroke mechanism for a scotch yoke assembly. The scotch yoke assembly of the present invention is comprised of a rotatable cam having a stroke length toggle eccentrically mounted to a surface thereof. Rotational movement of the cam causes the stroke length toggle to assume either a first or a second position depending on the direction of rotation. The first position defines a first stroke length which is shorter than a second stroke length defined by the second position.
The improved scotch yoke mechanism of the present invention uses fewer parts than conventional mechanisms, and is easier to adjust. Notably, the stroke length may be adjusted simply by reversing the rotational direction of the cam.